1. Field of the Invention
The present invention relates to a glass substrate for magnetic disks.
2. Discussion of the Background
A magnetic disk has a magnetic layer and a protective layer formed on a substrate by various processes including sputtering, plating, vapor deposition or the likes Generally, a glass is noteworthy as a magnetic disk substrate material suitable for highly densifying since it has an excellent surface smoothness, a satisfactory hardness and a high flow stress (resistance to deformation) and its surface defect is very few.
When a relatively cheap alkali-containing glass such as soda-lime silica glass is used as a glass substrate, it is known to corrode or discolor a magnetic film since an alkali ion is precipitated from a pinhole or peripheral part of a magnetic film or a glass-exposed part, particularly when it is under high moisture circumstances or it is subjected to aging treatment.
In order to avoid this problem, it may be proposed to use a glass of extremely low alkali content such as a non-alkali glass.
However, it is difficult to obtain a strength satisfactory for practical use when a non-alkali glass is used. It is because an ion exchange-strengthening treatment can not be carried out. Examples of strengthening treatments applied to a glass in order to obtain a strength satisfactory for practical use, include strengthening by air-cooling, ion exchange-strengthening, and the like, but in order to maintain a satisfactory flatness of a thin glass substrate for a disk, it is necessary to employ the ion exchange-strengthening treatment which can be conducted at a low temperature. The ion exchange-strengthening treatment is a treatment of exchanging an Na.sup.+ ion in an Na-containing glass with a K.sup.+ ion having a larger ion radius at a temperature not higher than the transition temperature of the Na-containing glass.
As mentioned above, it is an alkali content contained in a glass that is ion exchanged in the ion exchange-strengthening treatment, and therefore a non-alkali glass can not be subjected to the ion exchange-strengthening treatment. Thus, a substrate glass must be an alkali-containing glass in order to be subjected to the ion exchange-strengthening treatment at a low temperature.
As mentioned above, such properties of (1) not causing degradation of a magnetic layer under high moisture circumstances or during aging and (2) having a strength satisfactory for practical use, as demanded for a glass substrate for magnetic disks, are opposite properties, and it is difficult to satisfy the two properties at the same time.